stm class: Fix module init return on allocation failure

stm class: Fix module init return on allocation failure

In stm_heartbeat_init(): return value gets reset after the first
iteration by stm_source_register_device(), so allocation failures
after that will, after a clean up, return success. Fix that.

Fixes: 119291853038 ("stm class: Add heartbeat stm source device")
Reported-by: Hulk Robot <[email protected]>
Signed-off-by: Wang Hui <[email protected]>
Signed-off-by: Alexander Shishkin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Cc: stable <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>